We report measurements of the correlations between electromagnetic field quadratures at two frequencies f1=7 GHz and f1=7.5 GHz of the radiation emitted by a tunnel junction placed at very low temperature and excited at frequency f1+f2. We demonstrate the existence of two-mode squeezing and violation of a Bell-like inequality, thereby proving the existence of entanglement in the quantum shot noise radiated by the tunnel junction.
Generation and control of non-classical electromagnetic fields is of crucial importance for quantum information physics. While usual methods for the production of such fields rely on a non-linearity (of a crystal, a Josephson junction, etc.), a recent experiment performed on a normal conductor, a tunnel junction under microwave irradiation, has unveiled an alternative: the use of electron shot noise in a quantum conductor [1]. Here we show that such a device can emit pairs of microwave photons of different frequencies with a rate as high as that of superconducting Josephson junctions [2]. This results in intensity fluctuations of the photon field at two different frequencies being correlated below the photon shot noise i.e. two-mode amplitude squeezing. Our experiment constitutes a fundamental step towards the understanding of electronic noise in terms of quantum optics, and shows that even a normal conductor could be used as a resource for quantum information processing.While a classical current generates a classical field [3], also known as a coherent state of light, it appears that when electron transport requires a quantum mechanical description, the random electromagnetic field that corresponds to current noise is non- * jean-charles.forgues@usherbrooke. Another recent experiment has demonstrated that photo-assisted noise may exhibit correlations between the power fluctuations measured at two different frequencies for an adequate choice of the excitation frequency [5]. Since this experiment was performed at a relatively high temperature T = 3 K with regard to the frequency range, 4 − 8 GHz, i.e. k B T ≫ hf , the power fluctuations were classical, meaning that the measured correlations corresponded to fluctuations of photon fluxes with many (∼ 40) photons emitted within an experimental detection window. Here we report the observation of similar correlations at very low temperature T = 20 mK and under weak excitation. We demonstrate that there still are correlations between power fluctuations measured at two frequencies, f 1 and f 2 , even when in the quantum regime k B T ≪ hf 1,2 and when the average number of photons observed within a detection window is smaller 1
We report the measurement of the fourth cumulant of current fluctuations in a tunnel junction under both dc and ac (microwave) excitation. This probes the non-Gaussian character of photo-assisted shot noise. Our measurement reveals the existence of correlations between noise power measured at two different frequencies, which corresponds to two-mode intensity correlations in optics. We observe positive correlations, i.e. photon bunching, which exist only for certain relations between the excitation frequency and the two detection frequencies, depending on the dc bias of the sample.
We report experimental evidence that the microwave electromagnetic field generated by a normal conductor, here a tunnel junction placed at ultra-low temperature, can be non-classical. By measuring the quadratures of the electromagnetic field at one or two frequencies in the GHz range, we demonstrate the existence of squeezing as well as entanglement in such radiation. In one experiment, we observe that the variance of one quadrature of the photo-assisted noise generated by the junction goes below its vacuum level. In the second experiment, we demonstrate the existence of correlations between the quadratures taken at two frequencies, which can be stronger than allowed by classical mechanics, proving that the radiation at those two frequencies are entangled. To cite this article: J.-C. Forgues, G. Gasse, C. Lupien, B. Reulet, C. R. Physique 6 (2016). (Inspired from previous works/Inspiré de travaux antérieurs [1, 2]) Rayonnement non classiqueémis par un conducteur cohérent Nous rapportons des preuves expérimentales que le champélectromagnétique micro-ondes généré par un conducteur normal, une jonction tunnel placéeà ultra-basse température, peut avoir un comportement non-classique. Nous démontrons l'existence de compression d'état ainsi que d'enchevêtrement dans cette radiation en mesurant les quadratures du champélectromagnétiqueà une ou deux fréquences de l'ordre du GHz. Dans une expérience, nous observons que la variance d'une quadrature du bruit photo-assisté généré par la jonction descend sous son niveau du vide. Dans une deuxième expérience, nous démontrons l'existence de corrélations entre les quadratures observéesà deux fréquences, corrélations qui peuvent etre supérieuresà ce qui est permis par la mécanique classique, prouvant que la radiationà ces deux fréquences est enchevêtrée. Pour citer cet article : J.-C. Forgues, G. Gasse, C. Lupien, B. Reulet, C. R. Physique 6 (2016).
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